Recently, as semiconductor devices such as flash memories have become finer, application of a structure called a charge-trap type as a flash memory has been under investigation. Representative examples of a charge-trap type flash memory are a flash memory called a MONOS that uses a stack structure of a Metal/SiO2/SiN/SiO2/Si substrate (MONOS structure or MONOS type flash memory) and a flash memory called a TANOS that uses a stack structure of a TaN/Al2O3/SiN/SiO2/Si substrate (TANOS structure or TANOS type flash memory) (Refer to Japanese Unexamined Patent Application Publication No. 2008-98641.)
However, the TANOS structure is disadvantageous in that the reliability thereof is low and a threshold voltage changes. When the threshold voltage changes, data is likely to be unable to be written or deleted, and thus an operation of a semiconductor device such as a flash memory may be negatively affected by the change in the threshold voltage.